With the development of mobile communications technologies, transmission rates that communications systems can provide are increasingly higher, and meanwhile, users also have increasingly higher requirements on the transmission rates that the systems can provide. To meet a demand of a general user for a system transmission rate, and meanwhile, to provide a service of a higher system transmission rate for some users, in a situation in which substantially increasing a configured bandwidth is avoided, the 3rd Generation Partnership Project (The 3rd Generation Partnership Project, 3GPP for short) introduces a carrier aggregation technology. In this case, a user equipment may simultaneously use multiple component carriers to perform uplink and downlink communication, thereby supporting high speed data transmission; when the demand of a user for the system transmission rate decreases, some component carriers may also be released, and meanwhile a resident carrier is retained, so that a released transmission resource may be provided for another user to use.
Carrier aggregation is further divided into intra-band carrier aggregation and inter-band carrier aggregation. For the intra-band carrier aggregation, multiple carriers are in a same band; therefore, coverage consistency of the intra-band carrier aggregation can be kept. For the inter-band carrier aggregation, two bands participating in aggregation are relatively far apart; therefore, a coverage difference is large. Generally, a low-band carrier (also referred to as a low-frequency carrier) has relatively large coverage, while a high-band carrier (also referred to as a high-frequency carrier) has relatively small coverage. The low-frequency carrier, having relatively large coverage, can well used for performing connection controlling. A main feature of the high-frequency carrier is that resources abound, so that a very large bandwidth can be allocated for data transmission; but the coverage of the high-frequency carrier is relatively small; as a result, a user equipment can easily move out of the coverage of the high-frequency carrier.
In the foregoing scenario, the relatively small coverage of the high-frequency carrier causes the user equipment to easily move out of the coverage of the high-frequency carrier, resulting in frequent handover or base station reselection. To avoid occurrence of the frequent handover or the base station reselection of the user equipment, a control plane and a data plane of the user equipment may be separated, that is, control signaling and service data may be not necessarily transmitted on a completely same carrier. A relatively stable low-frequency carrier is used to transmit the control signaling of the user equipment, and only the high-frequency carrier is used to transmit data. However, if all data is transmitted on the high-frequency carrier, insufficiency of transmission resources of the high-frequency carrier is caused. Therefore, how a user equipment can fully use a high-frequency carrier and a low-frequency carrier to perform data transmission becomes a problem.